When inspecting a sample such as a semiconductor wafer using an electron beam inspection apparatus, a pattern shape image or an electrical defect image is obtained by controlling an accumulation amount of charges (hereinafter referred to as charging) to a pattern portion formed over a surface of the wafer. In particular, when the defect image is to be obtained, a contrast image (hereinafter referred to as a potential contrast image) caused by charged potential difference generated between a defect part and a normal part is required, and therefore, charge accumulation amount control (charging control) being uniform and having high accuracy is necessary. In order to perform charging control by electron beam irradiation, grounding of the wafer or voltage application to the wafer is necessary. However, if the wafer surface is covered with an insulation film such as an oxide, the insulation film is charged in excess if the grounding is insufficient, and distortion of the pattern shape image by charging and poor contrast of the potential contrast image are caused. Therefore, it is necessary to ground the wafer, and as a method thereof, a method of grounding the wafer by contacting a ground electrode to the wafer surface and preventing excess charging is conventionally known. For example, when the ground electrode is made to contact with the wafer surface, a contact resistance generated between the wafer covered with the insulation film and the ground electrode becomes a problem. However, there are a method of reducing a contact resistance value by carrying a constant current (surface activation current) between the ground electrode and the wafer as disclosed in Japanese Patent Application Laid-Open Publication No. 2006-13049 (Patent Document 1), and an antistatic method in which an electron beam is irradiated to a portion to be unnecessary in the wafer surface, local charging is generated to cause breakdown at a peripheral edge portion of the wafer, grounding to a portion of the breakdown is made, and as a result, the contact resistance is reduced as disclosed in Japanese Patent Application Laid-Open Publication No. 3-194915 (Patent Document 2).